A high voltage cathode of Na2+2xFe2−x(SO4)3 intensively protected by nitrogen-doped graphene with improved electrochemical performance of sodium storage

Abstract

As a high-voltage and earth-abundant element, in recent years, alluaudite, Na_2+2xFe_2−x(SO₄)₃, has been regarded as a highly promising cathode material of sodium ion batteries with higher energy density. However, the critical environmental sensitivity and limited conductivity of this kind of sulfate-based (SO₄²⁻) polyanionic material has led to its poor crystal stability and inferior intercalation ability. Herein, we report the design of nitrogen-doped graphene under low temperature conditions as an evolutionary modification approach to prepare the Na_2+2xFe_2−x(SO₄)₃; namely, an alluaudite sulfate Na_2+2xFe_2−x(SO₄)₃@N-rGO composite was prepared by a facile co-precipitation method assisted by the nitrogen-doped graphene. It is therefore surprising that the three-dimensional graphene-based network provides continuous electron pathways; thus, the Na_2+2xFe_2−x(SO₄)₃@N-rGO composite exhibits improved electronic conductivity and excellent sodium insertion capability, as well as the electrochemical performance. As a result, it delivers a reversible capacity of 93.2 mA h g⁻¹ with average redox potential of 3.8 V (vs. Na⁺/Na) at 0.05C; when the discharge rate increased to 10C, it delivers 56.3 mA h g⁻¹ and an amazing capacity retention of 83% is achieved after 400 cycles. On the other hand, the doped nitrogen species plays a huge role on improving the electron-donating ability of the graphene layer, which effectively protects the easily oxidized host material from deterioration, giving the material longer stability in a normal oxygen-containing atmosphere. We believe that this work may lead to a promising, low cost, suitable sodium ion battery material for next-generation large-scale energy storage devices.